The second generation cordless telephone (CT-2) Common Air Interface (CAI) describes the protocol for transmitting and receiving digitized audio and data for second generation cordless telephones. A CT-2 handset receives an analog speech signal via a microphone, converts the analog speech signal into a digital speech signal, compresses the digital speech signal, modulates the compressed signal at a radio frequency, and transmits the modulated RF signal through an antenna. The transmitted RF signal is received by a nearby base station where it may be converted to an analog signal or remain in digital form for digital switching equipment. Ultimately, the signal is relayed to the destination telephone. When a similar signal is received from the destination telephone, the telephone signal undergoes the same process in reverse. The base station transmits a digital RF signal to the handset which is received via the antenna demodulated, decompressed, and converted into an speech signal which drives a speaker in the handset.
The data is transmitted in one millisecond every two milliseconds with each frame containing four-bit adaptive differential pulse code modulated (ADPCM) samples and either two or four bits of data. Transmit and receive signals are sent and received to and from the base station in packets in a ping-pong fashion. CAI specifies that a part of the packet includes signalling information including such things as call setup and termination requests and handshaking information. For signal reception, a packet is received, demodulated, processed through a ADPCM decoder, converted from PCM to analog, and then provided to a speaker.
In the CT-2 cordless telephone system, the handset establishes a digital link with the base station. The link is normally maintained until the call is In some cases, the link may be lost or broken between handset and the base station during the call. For example, if the user moves the handset outside of range of the base station, then the link may be lost. The result is that a high level of white noise is provided the speaker due to the randomness of the received until the broken link is detected and the noise is muted The noise is extremely disturbing to the user at both the conventional telephone or the handset earpiece.
Normal analog systems rely on receive signal strength detection to squelch the audio circuits. Unfortunately, in digital systems such as CT-2, loss of carrier is not the only source of lost data. In addition, loss of frame synchronization will also cause the link (and the data) to be lost. Frame synchronization of the received signal is established only at the time of linkup. Once the link is established, the link is lost if sync is lost. If synchronization is lost, the received data becomes meaningless and random. Since data, though random and meaningless, is received, the use of carrier detect as squelch control is precluded because loss of synchronization may occur during acceptable signal strength levels. Random data received by an ADPCM codec to convert a signal to analog audio will translate as white noise at a level typically about 3 dB to 6 dB below the maximum level.
Presently, the only method which the CAI protocol uses to detect loss of link is the lack of a handshake message. Intervals in the transmission of handshake be as long as several seconds. One way to lessen the detection time is to force periodic communication between the base station and the handset, which results in a reduction in detection time to several hundred milliseconds. Even then, a user may hear a loud noise when the link is broken. Forced `D` (control) channel communication will give muting interval times on the order of hundreds of milliseconds. This is due to the fact that codewords are 80 bits in length and transmitted at 1 kbit/sec (mux 1.2) or 80 msec/codeword. Since a single bit will render the codeword invalid (CRC error), the algorithm would require a minimum of two to three codewords lost before muting (bit errors of 1% to 2% would cause only a slight clicking noise in the audio channel).
Thus, what is needed is a noise detector suitable for use in an RF communications device for quickly detecting a broken RF link so that audio output from the RF communications device can be muted with a minimum of audio noise.